Sediments from southern Hydrate Ridge show small fluctuation in grain-size distribution dominated by fine-grained (clay and silty clay) sequences locally interbedded with clayey silt to silty layers. Remarkable differences in sediment texture are only observed when specific intervals (Horizons A, B, and B´) are crossed. In general, sediments from southern Hydrate Ridge are fine grained characterized by an average mean size of 8.4
. Statistical parameters indicate poorly sorted sediments with an average standard deviation of ~2 and skewness of –0.08 corresponding to symmetrical curves. Calcium carbonate concentrations are generally low and range between 2.5 and 10 wt% (average = 6 wt%). High values as much as 25.9 wt% are associated with authigenic carbonates near the summit of southern Hydrate Ridge.
Bulk and clay mineral composition does not present significant changes between the summit and the flanks of the southern Hydrate Ridge sites. The bulk mineralogy is dominated by clays (30%–60%), quartz (25%–40%), feldspars (10%–25%), and minor amounts of calcite, which is usually below 5%. In the uppermost sampled sediments (2.5–16 mbsf) as well as above the BSR on the summit and east flank of southern Hydrate Ridge, the calcite content is as much as 20%. The clay mineral associations are dominated by detrital mica (average = 50%) and minor amounts of smectites, kaolinite, and chlorite ranging from 10% to 30%. Some noticeable trends are recognized in the smectite content, which usually increases downhole and below the BSR.
The main identified lithofacies comprise hemipelagic sediments, turbidites and ash layers, and debris flow deposits. The existence of coarser-grained horizons such as the ash-rich Horizon A may have played an important role in gas migration and gas hydrate formation (Tréhu et al., 2004a) as coarser-grained deposits are characterized by different packing structures and have a higher permeability that allow gas migration.
